Wireless communication devices, such as base stations and terminals, have a transmit chain which includes a power amplifier to amplify a modulated signal to a high power level for transmission over a wireless channel. It is known that elements in the transmit chain can introduce distortion to the transmitted signal and therefore there have been various proposals to compensate for distortion. One such proposal is a pre-distortion architecture where a low power modulated signal is pre-distorted in a manner which will compensate for non-linear effects of a power amplifier, before being applied to the input of the power amplifier. The combination of the pre-distortion applied to the input signal, and the (inevitable) non-linear distortion applied to the input signal by the power amplifier, result in a substantially distortion-free output signal.
One type of adaptive pre-distortion architecture, described in U.S. Pat. No. 6,275,685, receives an RF input signal and applies pre-distortion to the RF input signal. The pre-distorted signal is then input to a power amplifier. A portion of the output (also at RF) is fed back to a comparator, which compares the input signal (before pre-distortion) and the output signal. The output of the comparator is used to modify the amount of pre-distortion.
Another type of adaptive pre-distortion architecture applies pre-distortion in the digital domain, before up-conversion to RF. Pre-distorted signals for In-phase (I) and Quadrature (Q) channels are digitally created at baseband, separately converted to analog, and then up-converted to RF by applying them to the I and Q branches of an IQ up-converter. A portion of the RF output signal is fed back to a comparison function to control the pre-distortion system. This feed-back path is known as an observation receiver, and can either down-convert a sampled portion of the RF output signal to an Intermediate Frequency (IF), or can down-convert a sampled portion of the RF output signal directly to baseband.
For the IF option, the sampled RF signal is converted to IF by mixing with a LO signal, and then a single ADC operating at a high sample rate samples the IF signal. The digital output of the ADC is then mixed with digital quadrature LO signals to generate digital baseband signals. As the mixing process is digital the resulting I and Q channels are very well matched and essentially it can be assumed that no IQ perturbations are added by the observation receiver. A disadvantage of this arrangement is that the ADC must operate at a very high sampling rate. Also, where the up-converter directly up-converts to RF, the LO required for the up-converter operates at a different frequency to the LO in the down-converter of the observation receiver and therefore requires an extra synthesizer and risks danger of spurious frequency generation.
An alternative option is to down-convert the RF signal to a baseband signal. The sampled RF signal is applied to the I and Q branches of a down-converter where it is mixed with a local oscillator signal and down-converted directly to baseband. The down-converted I and Q signals are separately converted to the digital domain by a pair of ADCs. This has the potential of reducing the cost of the observation receiver, as the ADCs can operate at a lower frequency and effective anti-alias filters are easier to achieve. However, an IQ down-converter (or up-converter) architecture can add their own impairments to the transmitted signal or observation signal. These impairments are due to differences in the I and Q paths, and can arise in a mixer, anti-alias filter or ADC parts of the down-converter. This has inhibited the use, and effectiveness, of an IQ ADC architecture in the observation receiver path. An IQ up-converter will typically add dc, gain offset and angle offset impairments resulting in LO leakage and quadrature images in the RF spectrum. Methods to correct for these up-converter imperfections are known but rely on no additional quadrature impairments being added in the measurement system used to correct them. If the observation receiver uses an IQ architecture then this is not the case. Even once the IQ errors in the up-converter are compensated for, the errors in the down-converter impair the observation signal used to control the RF amplifier predistortion and limit the effectiveness of the amplifier predistortion correction loop. Accordingly, it is necessary to correct for errors introduced by the up-converter and down-converter.
The present invention seeks to provide a method to correct the quadrature impairments of a transmit chain having an IQ architecture in both the up-converter and the observation receiver.